For a given airspeed, an aircraft may consume less fuel at a higher altitude than it does at a lower altitude. In other words, an aircraft may be more efficient in flight at higher altitudes as compared to lower altitudes. Moreover, bad weather and turbulence can sometimes be avoided by flying above such weather or turbulence. Thus, because of these and other potential advantages, many aircraft are designed to fly at relatively high altitudes.
As the altitude of an aircraft increases, the ambient pressure outside of the aircraft decreases and, unless otherwise controlled, excessive amounts of air could leak out of the aircraft cabin causing it to decompress to an undesirably low pressure. If the pressure in the aircraft cabin is too low, the aircraft passengers may suffer hypoxia, which is a deficiency of oxygen concentration in human tissue. The response to hypoxia may vary from person to person, but its effects generally include drowsiness, mental fatigue, headache, nausea, euphoria, and diminished mental capacity.
Aircraft cabin pressure is often referred to in terms of “cabin pressure altitude,” which refers to the normal atmospheric pressure existing at a certain altitude. Studies have shown that the symptoms of hypoxia may become noticeable when the cabin pressure altitude is above the equivalent of the atmospheric pressure one would experience outside at 8,000 feet. Thus, many aircraft are equipped with a cabin pressure control system to, among other things, maintain the cabin pressure altitude to within a relatively comfortable range (e.g., at or below approximately 8,000 feet) and allow gradual changes in the cabin pressure altitude to minimize passenger discomfort.
To maintain aircraft cabin altitude within a relatively comfortable range, cabin pressure control systems may be equipped with one or more outflow valves. An outflow valve can assist in controlling cabin pressure by regulating air flow out of the cabin. One particular type of outflow valve that may be used is a butterfly outflow valve. A butterfly outflow valve typically includes a flapper or gate, which is typically used as the control element to regulate the flow of air out of the cabin. More particularly, the flapper is coupled to a shaft that is rotationally mounted to the outflow valve body. An actuator, which is coupled to the shaft, positions the flapper element in response to commands from a controller to thereby regulate the air flow out of the cabin.
Recently, there has been a move toward providing outflow valves that are relatively lightweight and compact, especially for the relatively newer generation lightweight and high altitude capable business and regional jet aircraft. Thus, it is desirable that the outflow valve body be constructed from a relatively strong, yet lightweight plastic or other composite material. Outflow valves constructed using these materials are generally safe, reliable, and robust. However, certain regulatory requirements require the outflow valve meet certain standards for postulated single point failures, such as a postulated crack in the outflow valve body. Because the crack propagation properties of certain plastics and other composite materials is not well defined, meeting these regulatory standards may not be possible.
Hence, there is a need for an outflow valve that is relatively strong and lightweight, is fairly simple to design and construct, and thus fairly inexpensive, and that can meet regulatory requirements for certain postulated single point failures. The present invention meets at least these needs.